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10

Chip Scale Review January • February • 2019

[ChipScaleReview.com]

ncredibly, the number of transistors

in integrated circuits (ICs) has

tracked Moore’s Law, doubling

every two years. Equally amazing is

the recent jump in IC device operating

frequency into the mmWave spectrum. The

emergence of 5G, next-generation WiFi

protocols, and automotive radar have pushed

ICs into these extreme frequency bands to

take advantage of the additional available

bandwidth. The need for instantaneous data

transfer has driven the increasing demand

for these new mmWave devices, whether

for safety concerns in automotive radar,

enormous data networks for 5G cellphone

backhaul, or simply the expectation to

instantaneously stream 4K video from your

tablet to your flat screen. Historically, 6GHz

was the high end of the frequency band for

the majority of ICs. Operating frequencies

have jumped to 30, 40, 60, and 80GHz to get

the necessary bandwidth for next-generation

5G, WiFi, and automotive devices. This

equates not to a doubling in frequency, but to

a gigantic leap that in some cases is greater

than an order of magnitude. An

additional test challenge beyond

the rapid growth in operating

frequencies is caused by 5G

applications driving an emerging

need for production over the air

(OTA) test of antenna in package

(AiP) devices (

Figure 1

).

Today, commercially-available

ICs already exist that require

bandwidth in the GHz range for

early 5G applications in various

mmWave f requency bands

from 30GHz to nearly 100GHz.

However, for years many of these

devices were only engineering

s a m p l e s c o n f i n e d t o a

characterization lab for in-depth analysis and

debug. Recently, mmWave devices, initially

from the automotive industry, are moving

quickly into the production environments

where high-volume testing is required. The

quick transition from concept to production

has created a bottleneck as device test plans

and interface hardware were previously

not yet fully defined. The high-volume

production test cells used for 6GHz devices

were modified to up-convert, mix, down-

convert, and measure mmWave frequencies.

This resulted in a very complex and custom

interface for each mmWave application.

Interface hardware has now become the

critical path for mmWave IC testing. What

historically was considered a high-speed

load board and spring pin socket no longer

provides sufficient performance for this new

generation of ICs. Now, the interface must

be considered holistically, including all the

mechanics of connecting the IC to the tester/

handler and the impact the environment has

on the transfer of data at incredible speeds.

In the mmWave frequency bands, miniscule

changes in the environment wreak havoc

on the electrical performance. Stack-up

tolerances and temperature fluctuations in

handler kits, docking hardware, connectors,

cabl i ng, et c., al l impact elect r ical

performance and the ability to get 100% test

coverage of mmWave devices.

Early on the expectation was to bring

mmWave devices to production with

the assistance of either loopback on the

load board or built-in self test (BIST) in

the device. Because tester resources for

mmWave frequencies were unavailable, test

engineers included loopback traces on the

load board from TX to RX, or IC designers

included mixing and couplers at the die

level to sample the mmWave signals without

interface to the outside world. Although

these methods circumvent the need for a

mmWave test system, it either becomes

too tedious to debug or consumes valuable

real estate and adds complexity to the die,

thereby delaying time to market and/or

pushing the IC cost upwards. Furthermore,

the accuracy and reliability of the test results

are not completely understood.

Testers now have first-generation up-

conversion, mixing, and down-conversion

add- on modu les t hat allow a more

traditional automatic test equipment (ATE)

test plan. These bolt-on modules extend the

frequency capability of the 6GHz tester to

the mmWave region. The typical maximum

bandwidth of these add-on modules is

around 10GHz so they are considered

banded solutions. Interface hardware can

be built to be broadband, but components

in the mmWave region are typically band

limited, which drives custom hardware

for different frequency bands.

A 5G module designed for

28-39GHz, a Wi Fi module

designed for 56GHz to 64GHz,

or an automotive radar module

designed for 76-81GHz require

a different add-on module for

the tester. Besides being banded,

these solutions are typically

scalar rather than vector to keep

costs down. They can measure

gain or output power, but not

phase. The full vector solution (a

vector network analyzer (VNA)

in a tester) systems exist, but

the increasing radio frequency

(RF) channel count of the new

devices are making full vector systems

prohibitively expensive.

Although most current generation testers

now have instrument options that can

supply at-speed signals to the device under

test (DUT), their calibration plane ends with

a power meter at the test head. That means

I

Production test interface solutions for mmWave and

antenna in package (AiP)

By Jason Mroczkowski, Dan Campion

[Cohu]

Figure 1:

Diverse 5G applications span the cmWave and mmWave bands.